Microphone cartridge with amplifier

ABSTRACT

The microphone of the invention obtains damping from a perforated baffle closely adjacent the diaphragm with fabric covering the perforations, and from resistive structures behind the baffle. The piezoelectric transducer element is mounted directly on a circuit board which also carries all electronic components of a preamplifier circuit.

United States Patent 1 3,660,602 Thompson [4 1 May 2, 1972 54] MICROPHONE CARTRIDGE WITH 3,440,363 4/1969 B611 ..317/235 M AMPLIFIER FOREIGN PATENTS OR APPLICATIONS I t P IT 7 Mmpm Cedar 1,076,182 2/1960 Germany ..179/180 [73] Assignee: Conrac Corporation, New York, NY. 1

' OTHER PUBLICATIONS [22] Filed: June 1,1970

- Carlson, A W1deband Mmlature Microphone, Journal of the App 78 Audio Engineering Society, p. 63 l- 635.

Primary E.\'aminer-KathIeen H. Claffy 2? 79/1 zzi g Assistant E.\'am1'ner.lon Bradford Leaheey nt. I02 Anomey chamon M Lewis [58] FIeIdoISearch ..l79/ll0A, l80;3l7/235 M;

310/82, 8.5; 340/ 57 ABSTRACT 5 1 R f Cited The microphone of the invention-obtains damping from a perforated baffle closely adjacent the diaphragm with fabric UNITED STATES PATENTS covering the perforations, and from resistive structures behind I Y the baffle. The piezoelectric transducer-element is mounted 2,791,64l 5/1957 Pye ..l79/I80 directly on a circuit board which also carries all electronic 3,130,275 4/1964 Hagey ..l79/l 10 A components ofa preamplifier circuit 3,497,731 2/l970 Straube... ..-....3l0/8.2 3,564,303 2/1970 Gell .L ..3 10/82 3 Claims, 6 Drawing Figures 36 i3 34 1 43 49 If ll 42 1 1 i 9.9 803 I 00 Z 94 b 60 i \II A J the entire microphone The microphonestructure of the invention-further provides particularly efi'ecti've and economical damping of the diaphragm movement; and facilitates'efi'ective protection of structure from external conditions such asdampnes's.

fullunde'rst'andingof the invention, and of its further-objects and'ad'vantages, will be had from thefollowingdescription of an illustrative structurefor carrying it out. The particulars of thatdescription,- and of the "accompanying drawings whichform part of it, are intendedonly as illustration and not as a limitation upon the scope .of the invention, which is defined in the appendedclaims.

In thedrawings: v FIG. l"isa-front' elevation' of a microphone unit illustrating the invention;

FIG. 2is a-section at enlarged scale on line 2-'-2 of FIG. 1; FIG. 3 is a detail section on line -3 3of FIG. 2; FIG. is a rear elevation, partially broken away; FIG. -5-is a fragmentarysection on line-5-5of FIG. 2 at an intermediatescale; and I FIG. 6 is a schematic diagram-representing illustrative circuitry. i g

The illustrative microphone {unit shown in the drawings comprises a housing 10of generally cylindrical form, closed at its rearend by-the wall l2and open at its front end at the circular edge 13; The housing wall is-deformed inwardly at two diametrically opposite sectors, forming internal shelf structu're's 14 which face toward theopenend'of the housing, and also formingthe extemal recesses 162i The disk 20' of dielectric material is mounted on. the two shelf'formations 1-4 by the eyelets 22 and 23 which also serve to mount external terminal lugs Hand 25' in; the respective recesses 16 and to connect them electrically to-internal'circuit structure tov be more fully described. Theplasti'c bushings 27 insulate eyelets'22 from the housingg whichis typicallya unitary metal die casting. Disk 20' is thickenough to be substantially rigid, and forms with housing end wall 1 2 a chamber 26.;Th'e'centra1 apertures 18 and 28 in housing wall l2and disk 20'} respectively, are covered by the pads 19 and 29" which' breathe sufficiently'to equalize atmospheric pressure; while sealingchamber 26 acoustically and: against--foreignmatter. In addition, aperture 28' and. pad- 29, form an acoustic resistance, and are of utility in shaping and controlling frequency response of the microphone. A

through the fabric. The degreeof'thatdamping action isreadily controllable by suitable'selection of such factors as the ratio ofbafile perforations to solid area, the bafile spacing from the diaphragm and the penneability of the fabric.

The transducer proper comprises a piezoelectric ceramic element 50 of bar form with its two active'longitudinal faces suitable mate'rialKfo'r those pads is the synthetic unw'oven fabric sold under the tradename Viskon. I

The soundresponsive diaphragm 30 -is slightly conical and is mounted against the front edge of the housing and is cemented thereto with-a suitable adhesive. The diaphragm and forward portion of the housing'are covered by the thinflexiblc membrane 32,- formedof a suitable plastic; which provides a substantially moisture impermeable barrier for the protection of the entire internal structure of the microphone. The diaphragm and moisture barrier are clamped'firmly in place by the gasket- 34 and the metal 'grille 36, the periphery of which is of cup form and grips the external housing flange 35.

The rigid metal bafile40 is of shallow conical form, closely matching the shape ofthe ;diaphragm,,and is mounted inward of the diaphragm in paralleland closely spaced relation. Baffle 40-is seated on-the 'intemalhousing flangedl and is retained by adhesive 43. It is generously perforated as indicated at 42. An air pervio'usfabric 44,- such as light silk cloth, is adhered to a face of the baffle',-covering perforations 42 and-offering a selected degreeof friction to airpassage through those perforations. Diaphragm vibration. in response to sound is damped by-the energy .loss-thataccompanies airCmovement coated with silver to render them conductive. Piezoelectric element 50 is ofatype that generates'an electrical potential between its conductivefaces when stressed by bending. Elev ment 50i's mounted onthe front face-of disk 20, oneend of the element-being held'effectively rigidly and the other end projecting freely and essentially radially into the central clearance aperture 28of the disk. That free end of element 50 is coupled to the center of diaphragm 30 by the drive pin 54, which projects inv spaced relation through the central clearance aperture 46. in baffle 40. One end of pin 54 is mounted semi-rigidly at the diaphragm apex by a drop of suitable adhesive 55; Theother end of the pin has an aperture which is pressed with a force fit over'the free end of element 50; which it grips firmly.

The electrical output signal from transducer element 50 is amplified by a suitable solid state amplification circuit incorporated in the microphone cartridge. An'illustrative circuit for that purpose is shown in schematic form at 60 in'FlG. 6, comprising the two ,transistors Qland Q2, directly coupled via the resistance R3. The amplified microphone output at tenninals 24 and 25 is shunted by capacitance C1, to bypass unwanted radio frequency energy often present when the microphone is used to modulate radiotransmitters. The dashed lines 72 in FIG. 6 represent -a flexible cable connecting the microphone cartridge output terminals 24 and 25 .to the, remainder of the electrical system. That system includes a power supply, indicated in schematic and illustrative form at A direct current power source is indicated at B, with the resistance R6, which acts as load impedance for transistor Q2. The terminals 74 and75 supply the microphone output to further amplification circuitry, forming part of the system remote from the microphone cartridge and typically of conventional design.

In accordance with one aspect ofthe present invention,- amplification circuitry 60 is constructed as a self-contained unit mounted on disk 20, already described, and including printed circuit connective elements formed directly on that disk as a circuit board. Disk 20 thus serves not only as mechanical support and electrical means of connection for transducer element' 50, but also as mounting support and essentially an integral part of the circuitry for amplifying the output from that transducer.

In accordance with the present invention, the various circuit components are mounted on the rear face of circuit board 20, so that in the assembled cartridge theyare contained within chamber 26. Those components are connected to form an amplifying circuit primarily by means of a conductive coating deposited in definitely predetermined configuration on the front face of circuit board 20, typically utilizing known printed circuit techniques. The wire leads originally provided on each circuit component typically project through small holes 82 in the circuit board and are connected to the printedcircuit coating by solder. The detailed arrangement of the components, and the exact configuration of printed circuit elements will depend, of course, upon the selected amplifying circuit.

An illustrative circuit arrangement is shown in somewhat fragmentary form in FIGS. 2 and 5.,Qwhere the circuit components that are mounted behind circuit board 20 are represented in FIG. 2 by R3 and Q2, and in FIG. 5 by R2, R3, Q1 and Q2 (shown in dotted lines), other components being omitted for clarity of illustration. Thus, as shown in FIG. 5, for example, transistor Q] has its respective terminals connected to printed elements a, 80b and 80c, while resistor R2 is connected between elements 80c and 80e. Printed elements 80d and 80e surround the holes in which mounting eyelets 22 and 23 are secured, and form electrical output connections via those eyelets to the respective output tenninals 24 and 25, already described.

A particular feature of the present invention concerns the manner of mounting ceramic piezoelectric transducer element 50. That element is .coated on its two working side faces in conventional manner with a conductive metal film. The mounted end of element 50 rests directly on printed circuit element 80a, with one of its coated faces making direct electrical contact with that element. The transducer is held effectively rigidly in that position by a spring which also makes electrical connection with the opposite coated face of the ceramic element. As shown, that spring 90 includes two spring arms 91 which are anchored at one end and have their opposite ends joined by the elongated bar portion 92. That bar is longitudinally corrugated to make it effectively rigid. The center of the corrugation rests on the ceramic element, securely clamping it to the circuit board and in reliable electrical contact with printed surface element 80a. The two anchored spring ends 91 rest directly on printed circuit element 80b and are secured by the eyelets 94, maintaining reliable electrical contact between the ceramic element and the circuit component Q1, for example.

In assembling the described structure, the entire amplifying circuit 60 is mounted on circuit board 20 together with transducer element 50, clamping spring 90 and driving pin 54. The electrical system can then be tested as a unit to any desired extent before mounting the circuit board in the housing. The circuit board assembly is mounted in the housing, secured by eyelets 22 and 23 and sealed around its periphery with suitable sealant 93.

Baffle 40 is then installed and diaphragm 30 is added, with pin 54 projecting through a small clearance hole at the center of the diaphragm. Only then is the pin coupled to the diaphragm by adhesive 55. That procedure relieves axial tolerances and insures unstrained equilibrium position of the moving parts.

I claim:

- 1. A microphone and amplifier unit, comprising in combination a generally cylindrical housing, open at one end to receiv sound energy,

a circuit board mounted transversely of the housing and having an outwardly opening central cavity,

an elongated piezoelectric element responsive to bending stresses and having two opposite conductive longitudinal output faces,

a conductive mounting surface on the outer face of the circuit board adjacent the cavity,

an electrically conductive spring clip having one portion rigidly mounted on the board and another portion resiliently engaging one face of the element to clamp the element with its other face engaging the mounting surface and with a free end extending unsupported over the cavity,

a sound responsive diaphragm peripherally mounted in spaced relation outward of the circuit board,

structure mechanically coupling the diaphragm to the free end of the element for driving the same,

and an amplifying circuit including circuit components mounted on the circuit board and printed circuit connecv tions on at least one board face interconnecting the components with said mounting surface and spring clip to supply the output of the element as input to the amplify- I ing circuit. 7

2. A microphone and amplifier unit as defined in claim 1,

and including also a substantially rigid perforated baffle plate peripherally mounted on the housing between the diaphragm and circuit board with a central clearance aperture for said couplingstructure,

and air pervious fabric covering the perforations of the baffle plate for damping the diaphragm movement.

3. A microphone and amplifier unit, comprising in combination a generally cylindrical housing, open at one end to receive sound energy, a circult board mounted transversely of the housing and having an outwardly opening central cavity,

an elongated piezoelectric element responsive to bending stresses and having two opposite conductive output faces,

structure mounting the element on the circuit board with a free end extending unsupported over the cavity,

a sound responsive diaphragm peripherally mounted in spaced relation outward of the circuit board,

a substantially rigid perforated baffle plate peripherally mounted between the diaphragm and the circuit board and having a central clearance aperture,

structure extending in spaced relation through said clearance aperture for mechanically coupling the diaphragm to the free end of the element to drive the same,

and an amplifying circuit including circuit components mounted on the circuit board and printed circuit connections on at least one board face interconnecting the components and cooperating with the element mounting structure to supply the output of the element as input to the amplifying circuit. 

1. A microphone and amplifier unit, comprising in combination a generally cylindrical housing, open at one end to receive sound energy, a circuit board mounted transversely of the housing and having an outwardly opening central cavity, an elongated piezoelectric element responsive to bending stresses and having two opposite conductive longitudinal output faces, a conductive mounting surface on the outer face of the circuiT board adjacent the cavity, an electrically conductive spring clip having one portion rigidly mounted on the board and another portion resiliently engaging one face of the element to clamp the element with its other face engaging the mounting surface and with a free end extending unsupported over the cavity, a sound responsive diaphragm peripherally mounted in spaced relation outward of the circuit board, structure mechanically coupling the diaphragm to the free end of the element for driving the same, and an amplifying circuit including circuit components mounted on the circuit board and printed circuit connections on at least one board face interconnecting the components with said mounting surface and spring clip to supply the output of the element as input to the amplifying circuit.
 2. A microphone and amplifier unit as defined in claim 1, and including also a substantially rigid perforated baffle plate peripherally mounted on the housing between the diaphragm and circuit board with a central clearance aperture for said coupling structure, and air pervious fabric covering the perforations of the baffle plate for damping the diaphragm movement.
 3. A microphone and amplifier unit, comprising in combination a generally cylindrical housing, open at one end to receive sound energy, a circuit board mounted transversely of the housing and having an outwardly opening central cavity, an elongated piezoelectric element responsive to bending stresses and having two opposite conductive output faces, structure mounting the element on the circuit board with a free end extending unsupported over the cavity, a sound responsive diaphragm peripherally mounted in spaced relation outward of the circuit board, a substantially rigid perforated baffle plate peripherally mounted between the diaphragm and the circuit board and having a central clearance aperture, structure extending in spaced relation through said clearance aperture for mechanically coupling the diaphragm to the free end of the element to drive the same, and an amplifying circuit including circuit components mounted on the circuit board and printed circuit connections on at least one board face interconnecting the components and cooperating with the element mounting structure to supply the output of the element as input to the amplifying circuit. 